The present invention concerns a storage rack for assemblages of nuclear fuel elements.
It is well known that nuclear reactors comprise assemblages of fuel elements disposed in the tank of the reactor following an appropriate configuration. These fuel elements are generally steel tubes filled with pellets of nuclear fuel materials. The fuel elements, which usually have a diameter of between 0.6 and 2 cm and a length of between 2 and 5.5 m, are grouped in "assemblages" which generally comprise at least several tens of fuel elements disposed in parallel fashion in bundles. The assemblages of fuel elements are assembled usually as prismatic structures having a square or hexagonal cross-section.
It is also known that after a certain period of functioning of the reactor, the irradiated fuel material should be withdrawn and replaced by new fuel material.
The spent fuel material is strongly radio-active and produces a considerable amount of heat. Indeed, it contains a residual quantity of initial fuel material and also numerous radio-active fission products.
It is important consequently to have a storage installation in order to store the assemblages of fuel elements which have been removed from the reactor, until they are either reutilized or sent from the nuclear center to a place of final storage or to another storage place.
Several types of storage installations have already been proposed and utilized in the past. As a general rule, such an installation consists of one or several "racks" in an adequate storage area, for example, in a pool filled with demineralized water or aqueous boric acid solution or in a storage area cooled by circulation of a gas or other appropriate fluid. The rack is composed of "chambers" each of which can receive and maintain an assemblage of fuel elements. Water plays simultaneously the roles of a cooling agent and of antiradiation shielding. The boron compounds which can be dissolved in the water of the pool, serve as "neutronic poisons" absorbing the neutrons.
Particularly a storage rack for assemblages of fuel elements (assemblages of square cross-section) is known in which each chamber is simply constituted by four angles disposed on the four corners of the chamber. All the angles which comprise the rack are held in place by being assembled to a series of traverses disposed between the chambers.
Also known is a storage rack composed of steel tubes, the cross-section of which corresponds to the form of the fuel assemblages which they must contain, said steel tubes being stayed and braced together by being fixed at their upper and lower parts to plates provided with holes of the appropriate dimensions and regularly spaced.
Other type of known storage racks are composed of steel tubes analogous to the above, cross-braced and tied to each other by being attached to metallic traverses disposed at several places between the rows of chambers.
It should be noted that in all these storage racks, the chambers are spaced from each other. It is important to take care that the fuel assemblages are separated by a sufficient distance in order to avoid a critical arrangement of nuclear fuel.
It is also true that some storage racks have been described as formed of adjacent storage chambers in checkerboard arrangement. However, in such storage installations, the spent assemblages of fuel elements can only be stored in one chamber out of two.
In fact, the storage installations for spent assemblages of fuel elements must comply with numerous very severe criteria, particularly concerning their strength and the precision of their dimensions, the security and ease of their use. These criteria imposed by the expert in the art, tend in addition to become more and more severe.